The goal of the proposed research is to develop a new and unique animal of neuroendocrine function useful for the study of the biological basis of mate choice or sexual partner preference. We have chosen the domestic ram because they display a naturally occurring variation in partner preference, such that 4-6% of range-bred populations prefer male sexual partners (i.e. male-oriented (MO) rams) in contrast to the majority of rams which prefer female sexual partners (i.e. female-oriented rams (FO) rams). Moreover, rams are dependent on gonadal hormones to facilitate sexual behaviors, and a great deal known about their sexual development and neuroendocrinology. Preliminary experiments, using rams that were characterized according to their sexual partner performances revealed that in comparison to FO rams, MO rams exhibited significantly lower levels of aromatase activity in the preoptic area (POA) of the brain, an area which is essential for the control of male sexual behaviors. In addition, our results suggest that MO rams may exhibit deficits in testicular hormone secretion. Further characterization and development of this ram model will provide a novel experimental animal system useful for studies in mental health, neuroscience, sexual behavior, neuroendocrinology and developmental neurobiology. The experiments proposed in this application will furnish important information that is needed to formulate and test novel hypotheses about the biological basis of sexual orientation in higher mammals including humans. For this purpose, we will perform the following specific aims: Specific Aim 1. To determine whether the expression MO sexual partner preference behavior is associated with altered function of the hypothalamic-pituitary-gonadal axis in rams. Specific Aim 2. To determine whether male sheep that exhibit MO sexual partner preference behavior have associated morphological differences in the size and neurochemical content of specific preoptic-anterior hypothalamic nucleic in comparison to FO males. Specific Aim 3. To determine MO sexual partner preference behavior can be artificially produced in genetic male sheep by providing male lamb fetuses of prenatal estrogen stimulation. When completed, this research will have characterized a new and unique animal model that will be useful for understanding the complex interactions between hormones, development, sensory processes, and behavior.